Wet gas purification method and system for practicing the same

ABSTRACT

A wet gas purification method is provided for removing ammonia from a heavy oil gasification gas, such as coal. The method principally involves a washing step during which absorbent is charged into the gas to absorb ammonia and an ammonia treating step wherein absorbent discharged from the washing step is separated into an effluent and an off-gas containing ammonia. The amount of absorbent charged during the washing step is controlled such that the ammonia concentration of gas exiting the washing step is 10 ppm or less.

BACKGROUND OF THE INVENTION

The present invention relates to a wet gas purification method forremoving ammonia from a gas. More particularly, the present inventionrelates to a wet gas purification method and a wet gas purificationsystem suitable for removing ammonia from a gas such as a coal or heavyoil gasification gas.

Conventionally, in a wet purification system for a coal or heavy oilgasification gas, sulfuric acid is added upstream of a hydrogen sulfide(H₂S) absorption tower as a pH adjuster. This allows for the removal ofammonia (NH₃), which is a source of fuel NOx in gas turbines, during thewater-washing step for removing impurities in the gas.

Specifically, if a gas containing ammonia is burned in a gas turbinewithout being purified, the gas becomes a NOx source, so that it isnecessary to recover as much ammonia as possible. To absorb the ammonia,a method for decreasing the pH of an absorbent in a water-washing toweris carried out, by which most of the ammonia can be removed. As a pHadjuster, sulfuric acid is preferably used in adjusting alkali ofammonia. The pH of the water-washing tower can be reduced by theaddition of the sulfuric acid.

The ammonia removed in the above-described water-washing step is takenout by an ammonia stripper. Since sulfuric acid is added to theabsorbent, ammonium sulfate exists in the discharged solution from theammonia stripper. The ammonia is recovered as aqueous ammonium sulfate.2NH₃+H₂SO₄→(NH₄)₂SO₄

To expel ammonia components from the aqueous ammonium sulfate and torecover ammonia as aqueous ammonia by a stripping method, caustic soda(NaOH) is added before the stripping. Usually, the caustic soda ischarged into a neutralization tank provided upstream of the stripper andis mixed. Then, the mixed liquid is sent to the stripper. The additionof NaOH yields sulfuric acid and ammonia again as described in thefollowing formula. Then ammonia is recovered.(NH₄)₂SO₄→2NH₃+H₂SO₄

On the other hand, in the above-described conventional system, one ofmajor reasons for adding sulfuric acid is to facilitate the operationcontrol of the system. That is to say, if the pH value of the liquid isdecreased by sulfuric acid, ammonia can be recovered and removed. Thereis no need for monitoring and controlling the amount of ammonia at thefollowing step of the water-washing step, and the control of pH sufficesfor operation. Because of such ease of operation, a method in whichsulfuric acid is added has been used.

Also, by decreasing the pH value, hydrogen sulfide is not removed in thewater-washing step, but passes through easily. A higher pH valuepresents a disadvantage of dissolution of hydrogen sulfide in theliquid, making the treatment of effluents complicated. Specifically, theconventional practice involves changing the pH value between theseparation/removal step of ammonia and the removal step of hydrogensulfide so that in the ammonia separation step, hydrogen sulfide doesnot dissolve in the liquid, and is not removed from the gas.

Moreover, the conventional practice requires the addition of sulfuricacid and caustic soda, which are costly. For example, to treat 1000 ppmof ammonia existing in the gas, ½ mole of sulfuric acid is needed forevery 1 mole of ammonia, which increases the chemical cost. Also, inorder to treat the nitrogen components in the effluent from awater-washing tower, which contains absorbed ammonia, at least 2 molesof caustic soda (NaOH) are required per 1 mole of sulfuric acid.Further, an amount of caustic soda equivalent to the amount of ammoniais needed. Therefore, the chemical expense is great. Thus, while theammonia can be recovered, the after-treatment steps are complicated andincrease in number.

Further, since the recovered aqueous ammonia accounts for about 20percent by weight, there is no other choice but to treat it as waste,which poses a problem of treatment cost.

SUMMARY OF THE INVENTION

In view of the above problems, the inventors carried out studies todevelop a method for purifying an ammonia-containing gas in whichchemicals such as sulfuric acid and caustic soda, which have beenregarded as necessary, are not used thereby significantly reducingoperation cost and effectively restraining emissions. Furthermore, thesteps and system are simple, operation is easy, and reliability is high.

As a result, the inventors arrived at the present invention throughtheir discovery that the above problems can be solved by continuouslymeasuring the concentration of ammonia in the gas after the washingstep, and controlling the charging makeup water in the washing step,rather than adding sulfuric acid to reduce the ammonia concentration to10 ppm or lower.

Specifically, the present invention provides a wet gas purificationmethod for removing ammonia in the gas comprising: a water-washing stepfor absorbing ammonia in the gas into absorbent to remove the ammonia;and an ammonia treating step for stripping the ammonia from theabsorbent to be discharged after the water-washing step to separate anoff-gas containing ammonia from effluents, wherein, makeup water ischarged continuously or intermittently in the water-washing step so thatthe concentration of the ammonia is 10 ppm or lower after thewater-washing step. The washing step may comprise a cooling step and acleaning step. That is to say, the partial pressure of NH₃ can bedecreased by decreasing the concentration of ammonia in aqueoussolution, rather than by actively controlling the pH value, to keep theNH₃ absorbing performance.

In the present invention, it is preferable that the method furthercomprises a step for burning the off-gas. By this off-gas burning step,hydrogen sulfide dissolved in the water-washing step can also bedecomposed simultaneously with the decomposition of ammonia.

Further, in the present invention, it is preferable that effluents inthe ammonia treating step be circulated charged to the washing step asmakeup water.

In the present invention, even if hydrogen sulfide is dissolved in theabsorbent, it can be separated to be removed from the liquid by astripper, so that there is no need for treating hydrogen sulfide in theliquid. The off-gas containing ammonia and hydrogen sulfide can betreated by being burned in the combustion furnace at a following step.It is efficient for the total system to burn the off-gas using aregenerative combustion furnace or a direct burning combustion furnacefor burning the off-gas coming out of a hydrogen sulfide absorbing step.According to the present invention, charging only makeup water can makeammonia stripping easy. Further, the discharged substance can be easilyused for combustion treatment.

According to the present invention, since only makeup water is charged,and neither sulfuric acid nor caustic soda is used, the cost requiredfor operation can be reduced significantly, and also the manipulationstep can be made simple and easy.

Since the makeup water is added, ammonia can exist in the form ofammonium carbonate (NH₄)₂CO₃ by the action of existing carbon dioxide.When the accumulated (NH₄)₂CO₃ is heated and the temperature rises, the(NH₄)₂CO₃ can release carbon dioxide to yield ammonia again. Bystripping ammonia NH₃ and carbon dioxide (CO₂) without using NaOH inthis manner, the cost required for operation can be reduced. Also, byburning H₂S and NH₃ in the effluent stripping gas, the cost for treatingaqueous ammonia can be eliminated.

The present invention is explained in detail below with reference to anembodiment. The scope of the present invention is not limited by thisembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the outline of a system suitable fora wet gas purification method in accordance with the present invention;

FIG. 2 is a schematic view showing a configuration of a purificationsystem used in Example 1; and

FIG. 3 is a graph in which the concentration of ammonia in a gasdischarged from the outlet of a cleaning tower is plotted against theamount of makeup water.

In the above figures, reference numeral 1 denotes a gas cooling tower, 2denotes a gas cleaning tower, 3 denotes a flash drum, 4 denotes anammonia stripper, 5 denotes an off-gas fired furnace, and 7 denotes acirculating pump.

DETAILED DESCRIPTION OF THE INVENTION

A specific embodiment of a wet gas purification method in accordancewith the present invention will now be described with reference to theattached drawings.

FIG. 1 is a schematic view showing one example of a system suitable fora wet gas purification method in accordance with the present invention.In the system of the embodiment, a water-washing step may comprise acooling step and a cleaning step. The cooling step may be performed by agas cooling tower 1, and the cleaning step may be performed by a gascleaning tower 2. Makeup water for absorbing ammonia components in a gasis continuously or intermittently charged into the gas cleaning tower 2.By these two water-washing towers, ammonia in the gas can be absorbed inan absorbent, and thereby removed. The amount of the makeup water to becharged is controlled so that the concentration of ammonia in the gas is10 ppm or lower after the water-washing step. The gas discharged fromthe gas cleaning tower 2 is sent to a hydrogen sulfide absorption tower.

In the washing step of the embodiment, ammonia in the gas is removed byadding makeup water without adding sulfuric acid for adjusting the pHvalue. The washing step (carried in the water-washing tower) consists oftwo steps, which use two towers: gas cooling tower 1 and the gascleaning tower 2. The makeup water charged into the gas cleaning tower 2can be circulated by a pump 7 and acts as the absorbent for absorbingammonia. The towers are coupled such that a part of the absorbent issent into the gas cooling tower 1 provided upstream of the gas flowdirection. A pump 7 circulates absorbent in the gas cooling tower 1 aswell.

Conventionally, in the gas cooling tower 1, a step of adding sulfuricacid has been carried out. In the embodiment, makeup water is introducedinto the gas cleaning tower 2 in place of sulfuric acid. Thereby, theconcentration of ammonia in the absorbent in the whole of the washingstep is decreased, and ammonia is removed. Specifically, when theconcentration of ammonia in the absorbent becomes high, the absorbenttends to release ammonia due to partial pressure. Thus, it becomesdifficult to remove ammonia using the absorbent in view of reactionequilibrium. Therefore, supplying makeup water continuously orintermittently can keep absorbent in a state of being able to absorb andremove ammonia.

Supplying makeup water in such a way allows the ammonia to react withcarbon dioxide in the gas to dissolve in the absorbent as ammoniumcarbonate. Ammonia is more easily stripped in this state, since the pHvalue of the absorbent is high compared with when sulfuric acid is alsopresent. Thus, ammonia and carbon dioxide can be taken out of theabsorbent by raising the temperature of an ammonia stripper provideddownstream of the water-washing towers without requiring the addition ofcaustic soda.

In the system the absorbent may be water (H₂O). The circulation of theabsorbent in the water-washing towers by the operation of the systemresults in the increased concentration of ammonia in the absorbent. Asdescribed above, in the embodiment, a gas cooling tower 1 and gascleaning tower 2 can be provided. By these two-towers, the concentrationof ammonia can be made low at the outlet of the gas cooling tower 1.Thus, water containing less dissolved components can always becirculated in the gas cleaning tower 2, so that ammonia can be absorbedto be removed effectively.

However, the washing step need not necessarily be provided with twotowers as in the embodiment. A water-washing tower consisting ofone-tower can also be used. Inside of the one tower can be divided intotwo, and gas cooling and gas cleaning can be performed in the one tower.In the tower, liquids at the upper part and lower part which can bedivided by a tray etc. are circulated separately.

The concentration of ammonia in the charged yielded gas is successivelymonitored, and an amount of makeup water best suitable for the amount ofammonia each time can be charged. The monitoring of the amount ofammonia can be performed, for example, by measuring the concentrationthereof in the yielded gas just before the gas cooling tower 1.

On the other hand, ammonia in a gas, which is charged into a gas turbinemust be reduced as much as possible because the ammonia is responsiblefor fuel NOx generation. In such case, the amount of charged makeupwater should be controlled to reduce the concentration of ammonia to 10ppm or lower. Therefore, it is usually preferable that the operation beperformed by monitoring the concentration of ammonia in the gas near theoutlet of the gas cleaning tower 2 at the later stage of water-washingstep and by checking that the concentration of ammonia has a value nothigher than a fixed value.

Absorbent is discharged from the water-washing step, some of which isusually drawn from the absorbent circulating in the gas cooling tower 1and sent to the ammonia stripper 4 via a flash drum 3. In the stripper4, ammonia is stripped from the absorbent. Then, the absorbent isdivided into an off-gas containing ammonia and the remaining washingliquid. The ammonia stripper 4 is usually operated at about 80° C. atthe upper stage and at about 130° C. at the lower stage.

Also, in this stripper 4, hydrogen sulfide components contained in theabsorbent are also removed, and are contained in the off-gas togetherwith ammonia. Therefore, the liquid after stripping does not containhydrogen sulfide. Such an off-gas containing ammonia and a minute amountof hydrogen sulfide is sent into an off-gas fired furnace 5, whereby theammonia and hydrogen sulfide are simultaneously burnt.

According to the above-described embodiment, the amount of effluent isincreased by the charge of makeup water. Therefore, it is preferablethat the effluent in the ammonia treating step be used by circulation asthe makeup water charged in the washing step.

A system in which the water coming out of the stripper 4 is used againas makeup water is advantageous because the amount of water treatedfinally as effluent does not increase as in the conventional example.Also, the system is preferable in terms of the ease with which makeupwater can be secured. Since the water discharged from the stripper 4usually has a temperature increased to 100° C. or higher, the water maybe supplied into the gas cleaning tower 2 as makeup water after beingcooled.

The gas to be purified in the present invention may be any gascontaining ammonia. Specifically, for example, the gas includes, but isnot limited to, coal gasification gas containing much ammonia andhydrogen sulfide.

The gas purification method in accordance with the present invention cansuitably used as a wet purification method for coal gasification gas atthe preceding step of a hydrogen sulfide removing step as part of asystem in which coal is gasified and used as power generation fuel. Bythe use of the above-described purification method in accordance withthe present invention, purification can be performed very efficiently inthe system for treating ammonia-containing gas.

According to the treating method in accordance with the presentinvention, since neither sulfuric acid nor caustic soda is used, thecost required for operation can be reduced significantly. Also, sinceammonia is accumulated as (NH₄)₂CO₃, NH₃ and CO₂ are stripped byincreasing the temperature without the use of NaOH, by which the costrequired for operation can be reduced.

On the other hand, HCN components, which are other harmful componentscontained in the gas, cannot be absorbed and removed unless theabsorbent has a high pH value. HCN components are scarcely removed by anabsorbent having a pH value of about 5 to 6. In the conventional methodin which sulfuric acid is added, the pH value of absorbent decreases (toabout pH 5 to 6). By contrast, in the present invention, the pH value ofabsorbent in the water-washing towers increases (to about pH 6 to 9).Therefore, according to the purification method in accordance with thepresent invention, the HCN removing performance is improved by theincrease in pH value of absorbent to about 6 to 9.

Also, according to the present invention, causes of material corrosioncan be reduced by the increase in pH value of absorbent in thewater-washing towers and the decrease in concentration of chlorine inthe absorbent due to the addition of makeup water.

The experimental results showing the advantageous effects of the presentinvention will now be explained in detail as an example. However, thepresent invention is not limited by the example.

EXAMPLE 1

FIG. 2 shows the outline of a system in example 1.

Two towers are provided: the gas cooling tower 1 is provided upstream ofthe flow of gas, and the gas cleaning tower 2 is provided downstream ofthe flow of gas. From the inlet of the gas cooling tower 1, a yieldedgas having an ammonia concentration (y0) of 1200 ppm was introduced at agas flow rate of 3500 m³N/h under a pressure of 0.9 MPa.

The amount (×kg/h) of makeup water charged into a circulation line ofthe gas cleaning tower 2 was changed continuously in the range of 40 to350 kg/h.

When the temperatures of both of the gas cooling tower 1 and the gascleaning tower 2 were set at 40° C., as shown in FIG. 3, the ammoniaconcentration (y2) measured at the outlet of the gas cleaning tower 2decreased as the amount of makeup water increased. The amount of makeupwater in this example indicates the amount in a preliminary plant. Fromthe results, it was found out that if the amount of makeup water chargedin the washing step is increased the concentration of ammonia in theyielded gas can be decreased very effectively.

The above is a description of the embodiment and example of the presentinvention. The embodiment and example are provided to aid inunderstanding the present invention, and do not limit the scope of thepresent invention.

1. A wet gas purification method for removing ammonia from a coal orheavy oil gasification gas, the method comprising: a water-washing stepcomprising a gas cleaning step and a gas cooling step which is carriedout upstream of the gas cleaning step in a gasification gas flowdirection, wherein an amount of absorbent make-up water for absorbingammonia is charged into the coal or heavy oil gasification gas in thegas cleaning step, part of the absorbent used in the gas cleaning stepis sent to the gas cooling step such that ammonia is absorbed into theabsorbent to be removed from the gasification gas in both the gascooling and the gas cleaning step, and at least a portion of theabsorbent make-up water is discharged from the washing step; and anammonia treating step for stripping ammonia from the absorbent make-upwater discharged after the water-washing step to separate the dischargedabsorbent make-up water into an off-gas containing ammonia and aneffluent, wherein the amount of absorbent makeup water charged in thegas cleaning step is controlled so that the concentration of ammonia inthe gasification gas after the gas cleaning step is 10 ppm or lower. 2.The wet gas purification method according to claim 1, further comprisinga step of burning the off-gas after the ammonia treating step.
 3. Thewet gas purification method according to claim 1, wherein the effluentin the ammonia treating step is circulated to the washing step asabsorbent make-up-water.
 4. A wet gas purification system for removingammonia from a coal or heavy oil gasification gas, the systemcomprising: water-washing towers comprising a gas cleaning tower with agasification gas outlet and a gas cooling tower which is locatedupstream of the gas cleaning tower in a gasification gas flow direction,the gas cooling and the gas cleaning tower being coupled such thatabsorbent make-up water charged and used in the gas cleaning tower issent to the gas cooling tower, allowing ammonia to be absorbed by theabsorbent make-up water in both the gas cooling tower and the gascleaning tower; and an ammonia stripper operatively connected to the gascooling tower for receiving absorbent make-up water discharged from thegas cooling tower, the ammonia stripper being configured for strippingammonia from absorbent discharged from the gas cleaning tower toseparate the discharged absorbent make-up water into an off-gascontaining ammonia and an effluent; wherein the concentration of ammoniaat the gasification gas outlet can be maintained at 10 ppm or lower bycontrolling the amount of absorbent make-up water charged in the gascleaning tower.
 5. The wet gas purification system according to claim 4,further comprising an off-gas fired furnace for burning off-gasdownstream of the ammonia stripper.
 6. The wet gas purification systemaccording to claim 4, wherein the effluent from the ammonia stripper canbe circulated to the water-washing towers as absorbent make-up water.